This invention pertains to a current supply circuit, and more particularly, to such a circuit which includes changeable-level voltage source for the supply of current, the level of which source is varied in accordance with the impedance of a load connected to the circuit. For the purpose of illustration herein, a preferred embodiment of the circuit is described in connection with transcutaneous stimulation which is used for pain relief purposes. In such apparatus, the instant invention has been found to have particular utility.
Speaking in very general terms, a conventional current supply circuit includes a current source connected in series with what is known as a compliance voltage source. These two devices are connected in series between a pair of output terminals adapted for connection to a load. Typically, the compliance voltage source operates at a particular maximum voltage, which voltage is divided between the current source and the output terminals directly in accordance with the impedance of any load connected to the output terminals. The larger the impedance value of a connected load, the more of the compliance voltage which appears across the load and the less of it which appears across the current source. The lower the impedance value of a load, the smaller the voltage across the load and the larger the voltage across the current source. The sum of the voltages across the load and the current source always substantially equals the maximum voltage level of the compliance voltage source.
While there are applications where this kind of an arrangement is entirely satisfactory, there are many others where it is desirable to reduce and limit the maximum voltage which can appear across output terminals in such a supply circuit. One of these applications concerns devices known as transcutaneous stimulators which, essentially, are pulsed current supply circuits intended for connection through electrodes to a person's skin for the purpose of creating electrical current nerve stimulation. Such a device is used quite frequently for pain relief. However, it is known that it is possible for too high a voltage between output electrodes to cause skin damage. Further, if too high a voltage exists between such electrodes in their open-circuited condition, then, if a stimulator, while turned on, is connected to a person's skin, a substantial shock can occur.
A general object of the present invention is to provide a current supply circuit which includes a changeable-level compliance voltage source whose voltage output level is modified in accordance with the impedance of a load connected to the circuit.
More particularly, the present invention proposes a circuit wherein, so long as the value of the impedance of a connected load is below a predetermined maximum value, the overall circuit performs essentially like an ordinary current supply circuit of the type generally described above. However, on the connected load exceeding this predetermined maximum impedance value, a control subcircuit, which is included in the circuit, responds to this condition quickly to shut down the level of the voltage in the compliance voltage source to an acceptable minimal level, which level remains until the impedance value of any connected load is again below the predetermined maximum impedance value.
With this kind of arrangement, the open-circuit voltage which exists between output terminals in such a supply circuit when the same is turned on, is the minimum voltage level just mentioned, and there is no likelihood of a shock occurring when electrodes are connected to a person. Further, if, when electrodes are connected, the skin impedance between the electrodes exceeds the predetermined maximum impedance value, the compliance voltage level is held at the minimum voltage level so that no skin damage can occur.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying single drawing figure.